1. Field of the Invention
The present invention is directed to a method and magnetic resonance MR apparatus of the type employing multiple coils for obtaining magnetic resonance signals, and in particular to a method and apparatus for calibrating the sensitivities of those coils.
2. Description of the Prior Art
Magnetic resonance signals are received with multiple coils in a magnetic resonance imaging apparatus in many types of magnetic resonance imaging sequences. These types of sequences are generally referred to as partially parallel acquisition (PPA) techniques, and include the known SMASH and SENSE sequences, as well as others. Details that are common to most PPA techniques are described in co-pending application Ser. No. 10/117,396, filed Apr. 5, 2002 (“Magnetic Resonance Imaging Method and Apparatus Employing Partially Parallel Acquisition,” Griswold), now U.S. Pat. No. 6,841,998, the teachings of which are incorporated herein by reference. In sequences employing PPA, an accelerated imaging acquisition is possible because magnetic resonance signals are being simultaneously received by multiple reception antennas (reception coils). The received signals are processed in parallel, however, it is important to have knowledge of the coil sensitivity of each reception coil in the processing of the received signals. Processing errors would occur if the signals were processed based on the assumption that all of the coil sensitivities are identical.
Currently, two methods are known for calibrating the coil sensitivities for this purpose.
In a first of these known methods, reference scans are performed in a separate measurement before the accelerated image acquisition. As schematically indicated in FIG. 1A herein, the reference scan can be a 2D reference scan conducted with the same slice orientation and position as each slice for which an image is to be obtained in the accelerated imaging. As shown in FIG. 1A, if images of two slices (Slice 1 and Slice 2) are to be obtained, a reference scan in this version is conducted preceding each accelerated imaging signal acquisition.
Alternatively, the reference scan can be a volume reference scan (3D reference) as indicated in FIG. 1B, and therefore a single volume reference scan can precede the respective accelerated imaging signal acquisitions for multiple slices.
A disadvantage associated with this method (both versions) is that the separation of the reference scan and the imaging scan in different signal acquisition measurements may cause coil sensitivity misalignment between the two types of scans, and thus artifacts may arise in the reconstructed images. Moreover, in real time magnetic resonance scans, the slice orientation and position may be adjusted interactively, and therefore it is not convenient, and may not be possible, to undertake a separate reference scan each time the slice is redefined. Data sharing between the reference scan or scans, and the imaging scans, introduces additional engineering workloads.
The second of the aforementioned known methods for calibrating coil sensitivities was developed to address these problems.
The second of these known methods for calibrating coil sensitivities is referred to as self-calibration. An example of this technique is AUTO-SMASH, as described in “V-D-AUTO-SMASH Imaging” Heidemann et al., Magnetic Resonance In Medicine, Vol. 45 (2001), pages 1066-1074 and “AUTO-SMASH: A Self-Calibrating Technique for SMASH Imaging,” Jakob et al., Magnetic Resonance Materials in Physics, Biology and Medicine, Vol. 7 (1988), pgs 42-54. Self-calibration is schematically indicated in FIG. 2 herein. In an accelerated imaging scan, a small number of additional k-space centerlines are acquired, and these lines can be used as reference scans for coil sensitivity calibration as well for imaging scans, to improve the signal-to-noise ratio. The reference scans and the accelerated imaging scans now share the same slice definition. The coil sensitivity information also is updated in each measurement to avoid the aforementioned coil sensitivity, misalignment problem. Nevertheless, there are several problems associated with self-calibration. First, the acquisition of additional k-space centerlines represents a substantial obstacle to improving data acquisition efficiently. Second, reference scans are limited by the imaging parameters that are defined for the accelerated imaging scans, such as field of view (FOV), bandwidth, etc. Third, the number of reference scans is limited so as to avoid any significant reduction in the actual acceleration factor due to the reference lines. Lastly, both the reference scans and the imaging scans must be conducted using the same type of magnetic resonance pulse sequence. This may compromise the coil sensitivity calibration, as shown in FIG. 3 herein.
FIG. 3 is a comparison between the coil sensitivity calibration using three types of sequences, namely spin-echo, FLASH, and TrueFISP. A total of 128 phase-encoding lines were measured for reference scans. A 100 Hz frequency offset was introduced to simulate the possible magnetic field inhomogeneity in the region of a human heart. The sensitivity of one coil element calculated using data from the spin echo sequence is employed as the “ideal” standard. The difference between the FLASH and spin echo sequence is shown as curve A. Curve B shows the difference between the TrueFISP and spin echo sequence. Clearly there is a much larger deviation in coil sensitivity calibrated by the TrueFISP sequence, which is more sensitive to the off-resonance effects than the FLASH sequence